1. Technical Field
The disclosure relates to a rubber composition for a tire tread used in a tread of a pneumatic tire, and a pneumatic tire using the rubber composition.
2. Related Art
A pneumatic tire is required to improve rolling resistance performance contributing fuel efficiency, and wet performance that is grip performance on wet road surface. However, those performances generally have paradoxical relationship, and it is difficult to simultaneously satisfy two characteristics of rolling resistance performance and wet performance. To simultaneously satisfy those performances, silica is added as a filler. The silica is generally difficult to disperse in a rubber component. Therefore, a silane coupling agent is added to improve dispersibility of silica. However, the dispersibility is still insufficient, and it cannot be said that the characteristics of the silica are sufficiently exerted.
To improve dispersibility of silica, an attempt is made to, for example, increase mixing energy, change a structure and a particle diameter of silica, or use a modified rubber comprising a diene rubber having introduced therein a hydroxyl group, an amino group or the like (for example, see WO96/23027 and WO03/029299).
Furthermore, JP 2009-108308A discloses that a lignin derivative such as lignin sulfonate is added to a diene rubber in order to improve dispersibility of silica, thereby improving rolling resistance performance.
In those prior arts, a certain extent of effect is recognized in the improvement of rolling resistance performance and wet performance due to the improvement of dispersibility of silica, but it cannot be said that the effect is sufficient.
On the other hand, JP 11-60816A (1999) discloses a rubber composition for a tire tread, comprising, for the purpose of improving rolling resistance performance and wet performance, an oil extended rubber component A comprising a styrene butadiene rubber having high glass transition temperature; a carbon masterbatch comprising a mixture of a rubber component B comprising a styrene butadiene rubber having low glass transition temperature, and carbon black, the masterbatch having a Mooney viscosity higher than that of the oil extended rubber component A; and silica. JP 11-60816A describes that the oil extended rubber component A containing silica and the rubber component B containing carbon black are unevenly distributed in an appropriately heterogeneous state in the rubber composition, while having appropriate compatibility, respectively, thereby exerting characteristics of each rubber component. However, the silica cannot be dispersed well in the oil extended rubber component A by only the difference in Mooney viscosity, and improvement effects of rolling resistance performance and wet performance are insufficient.